1. Field
One embodiment of the present invention relates to a method of manufacturing a magnetic recording medium having discrete tracks on the surface of a magnetic recording layer and, more particularly, to an ultraviolet-curable resin material to be used when transferring a discrete track shape.
2. Description of the Related Art
Recently, the nano-imprinting techniques are attracting attention in various fields in order to further increase the density and accuracy.
For example, applications to semiconductors, optical elements, magnetic recording media, and the like are being examined.
As a magnetic recording medium, a discrete track medium is attracting attention. In this discrete track medium, magnetic interference between adjacent recording tracks is reduced by separating the adjacent tracks by grooves or guard bands made of a nonmagnetic material in order to further increase the density.
When manufacturing this discrete track medium, discrete track patterns of a magnetic layer can be formed by applying the nano-imprinting technique by using a stamper. When magnetic layer patterns corresponding to servo area signals are formed together with recording track patterns by imprinting, it is possible to obviate the servo track writing step required in the manufacture of the conventional magnetic recording media. This leads to a cost reduction.
As the process of forming discrete track patterns as described above, a process of transferring resist patterns from an Ni stamper by, e.g., high-pressure imprinting or thermal imprinting has been used. Unfortunately, this process is unsuitable for mass-production because the life of the Ni stamper is short. Also, when the data density is increased to make tracks finer, resist patterns cannot be well transferred.
From the foregoing, the use of optical nano-imprinting is attracting attention as another nano-imprinting technique.
To transfer patterns onto a resist on a discrete track medium by using optical nano-imprinting, a resin stamper is first duplicated from an Ni stamper (mother stamper) by injection molding, and bonded in a vacuum to an uncured ultraviolet-curable resin layer to be used as a resist. This method is found to be able to reduce the cost and suitable for micropatterning.
The characteristics required of the ultraviolet-curable resin to be transferred onto the above-mentioned discrete track medium are the property of coating onto the medium, the viscosity, the curing property, the property of separation from the resin stamper, the resistance against etching for processing transferred patterns, and the cure shrinkage. The thickness of the ultraviolet-curable resin must be sufficient for imprinting with respect to the height of the three-dimensional structure of transfer patterns. For later processing steps, however, the amount of residue of the ultraviolet-curable resin after imprinting is preferably small. Therefore, the coating film thickness of the ultraviolet-curable resin layer is desirably 60 nm or less.
An example of the ultraviolet-curable resin for radical polymerization is an ultraviolet-curable resin obtained by mixing an initiator, an oligomer having a vinyl (acryloyl) group, and a monomer (see, e.g., non-patent reference 1). However, when an oligomer is mixed in an ultraviolet-curable resin, the viscosity increases, and this makes it difficult to decrease the coating film thickness to 60 nm or less.
Also, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2008-19292, an example of the ultraviolet-curable resin for nano-imprinting is an ultraviolet-curable resin to which a surfactant is added to improve the coating property and the property of separation. If the amount of surfactant is too large, however, curing inhibition readily occurs, the curing time tends to prolong, or the magnetic recording medium often deteriorates.
In addition, since the coating film thickness must be very small, i.e., 60 nm or less, film thickness control is difficult to perform unless the viscosity of the ultraviolet-curable resin is 15 cP or less.
If a monofunctional monomer alone is cured, the curing property of the film degrades. On the other hand, if the functional order is increased, the film cures, but the cure shrinkage readily increases.